Superconducting materials have already been practically applied in the form of a superconducting magnet in a particle accelerator, a medical diagnosing instrument and the like. Potential applications of the superconducting materials include, an electric power generator, an energy storage device, a linear motor car, a magnetic separator, a nuclear fusion reactor, a power transmission cable, and a magnetic shielder. In addition, a superconducting element using the Josephson effect is expected to be applied in such fields as an ultra-high speed computer, an infrared sensor and a low-noise amplifier. The magnitude of the industrial and social impact which would be exerted the practical realization of these possible applications is really unmeasurable.
One of the typical superconducting materials so far developed is an Nb-Ti alloy which is widely used at present as a magnetizing wire. The Nb-Ti alloy has a critical temperature, i.e., a critical temperature from which a superconductive state occurs (hereinafter simply referred to as "Tc") of 9.degree. K. As a superconducting material having a "Tc" considerably higher than that of the Nb-Ti alloy, a compound-type superconducting material has been developed, including an Nb.sub.3 Sn (Tc: 18.degree. K.) and V.sub.3 Ga (Tc: 15.degree. K.) which are now practically employed in the form of a wire.
As a superconducting material having a "Tc" further higher than those of the above-mentioned alloy-type and compound-type superconducting materials, a composite oxide superconducting material containing a Cu.sub.x O.sub.y -radical has recently been developed. For example, a Y-Ba-Cu-O type superconducting material has a "Tc" of about 93.degree. K. Since liquid nitrogen has a temperature of 77.degree. K., liquid nitrogen available at a lower cost than liquid helium can be used as a cooling medium for the composite oxide superconducting material. Discovery of a superconducting material having a high "Tc" applicable at a temperature of liquid nitrogen urges further expectations for the foregoing fields of application. In the actual application, however, problems are how to process a superconducting material in the form of a film or a wire, and at the same time, how to increase a critical current density (hereinafter simply referred to as "Jc") of the superconducting material.
In order to increase the "Jc" of a superconducting material, it is necessary, when using the superconducting material in the form of a film, to make the structure of the film dense with a single superconducting phase.
A method for manufacturing a superconducting article, in which "Jc" of a film of a superconducting material can be increased by making the structure of the film of the superconducting material dense with a single superconducting phase, is disclosed in the "Japanese Journal of Applied Physics", Vol. 27, No. 8, pages L1501-L1503, published on Jul. 22, 1988 (hereinafter referred to as the "prior art"). The prior art is described below with reference to the drawings.
FIG. 1 is a schematic descriptive view illustrating the former half steps of the method of the prior art for manufacturing a superconducting article, and FIG. 2 is a schematic descriptive view illustrating the latter half steps of the method of the prior art for manufacturing the superconducting article. First, a sheet-shaped substrate 1 comprising Y.sub.2 BaCuO.sub.x' is prepared. Then, a mixture of CuO and BaCO.sub.3, in which the ratio of copper (Cu) to barium (Ba) is Cu:Ba=5:3 in molar ratio, is primary-fired at a temperature of 800.degree. C. for 24 hours, cooled, and pulverized into a powder. The powder of the thus primary-fired mixture is then secondary-fired at a temperature of 900.degree. C. for 24 hours, cooled, and pulverized into a powder to prepare a powdery material for a film. Subsequently, the thus prepared powdery material for a film is mixed with ethyl alcohol to prepare a slurry for a film.
Then, the thus prepared slurry for film is applied onto the surface of the substrate 1, and dried to form a film 2 comprising Ba-Cu oxides on the surface of the substrate 1, as shown in FIG. 1. Then, the substrate 1, on the surface of which the film 2 has thus been formed, is heated in an electric furnace to melt the film 2 to cause the resultant melt of the Ba-Cu oxides in the film 2 to diffusion-react with Y.sub.2 BaCuO.sub.x' in the substrate 1, thereby converting the film 2 into a film 3 of a superconducting substance comprising YBa.sub.2 Cu.sub.3 O.sub.x, as shown in FIG. 2.
Then, the film 3 of the superconducting substance thus produced is cooled to a room temperature, thereby manufacturing a superconducting article comprising the non-reacting substrate 1 and the film 3 of the superconducting substance formed on the surface of the nonreacting substrate 1, as shown in FIG. 2.
The above-mentioned prior art has the following effects: Since the film 3 of the superconducting substance comprising YBa.sub.2 Cu.sub.3 O.sub.x is produced through the diffusion-reaction of the resultant melt of the Ba-Cu oxides in the film 2 with Y.sub.2 BaCuO.sub.x' in the substrate 1, the structure of the film 3 of the superconducting substance is dense with a single superconducting phase, thus permitting manufacture of a superconducting article having a high "Jc".
However, the above-mentioned prior art has the following problem: When the film 3 of the superconducting substance comprising YBa.sub.2 Cu.sub.3 O.sub.x is produced on the surface of the substrate 1 through the diffusion-reaction of the resultant melt of the Ba-Cu oxides in the film 2 with Y.sub.2 BaCuO.sub.x' in the substrate 1, the film 3 of the superconducting substance expands in volume, causing cracks in the film 3 of the superconducting substance and resulting in seriously deteriorated superconducting properties of the superconducting article including a largely decreased "Jc".
The above-mentioned problem occurs also in the case where the film 3 of a superconducting substance is produced by means of a compound containing an optional rare earth element other than "yttrium" (Y) in the above-mentioned Y.sub.2 BaCuO.sub.x' and YBa.sub.2 Cu.sub.3 O.sub.x. Such an optional rare earth element is hereinafter represented by "Ln".